6x12 Coconut Shell Activated Carbon for Gold Recovery: Iodine Number, Hardness and CIP/CIL Fit

By the YELI Technical Team - Updated October 2026 - 9 min read

Short answer: For CIP/CIL gold recovery, order 6x12 mesh coconut shell granular activated carbon with iodine number 1000-1300 mg/g, hardness at least 98% (ASTM D3802 ball-pan), platelet content below 1.5%, moisture at most 5%, ash at most 5% and apparent density 0.45-0.52 g/cm3, and require a batch COA that reports those numbers on the lot actually shipped. 6x12 mesh is the default size for gold plants because it is coarse enough to be retained by interstage screens and fine enough to keep adsorption kinetics fast. Iodine number tells you how much pore capacity you are buying; hardness tells you how much of it - and how much gold - you will still have after 200 transfer, elution and regeneration cycles. Buyers who specify only one of the two normally find out the hard way.

Activated carbon is the cheapest reagent in a gold plant and one of the easiest to buy badly. In CIP (carbon-in-pulp) and CIL (carbon-in-leach) circuits the carbon is both the adsorption medium and a consumable moving through pumps, screens and columns, so the specification has to satisfy two different jobs at once: adsorb gold cyanide fast, and survive the circuit long enough to be eluted instead of lost. This guide covers the 6x12 specification in the detail a procurement engineer actually needs - mesh, iodine number, hardness, density, ash - and how to check all of it against a real certificate of analysis.

1. Why 6x12 mesh is the standard cut for CIP/CIL

6x12 mesh (also written 6×12 mesh, the form used on our specification sheet and in YS/T 3038-2020 references) corresponds to approximately 1.70-3.35 mm particle size (GB/T 12496.2 / ASTM D2862). It is the most widely used cut in gold recovery because it sits at the balance point of three competing needs:

• Retention. CIP and CIL tanks use interstage screens, and carbon that passes the screen reports to tails with the gold still on it. Coarser carbon is easier to retain.
• Kinetics. Gold adsorption is a diffusion process. Smaller particles shorten the diffusion path and reach equilibrium gold loading faster, which matters most in the last adsorption tank where the solution gold grade is lowest.
• Hydraulics and attrition. In columns and transfer lines, very fine carbon raises pressure drop; in pulp tanks it abrades faster and generates the fines you lose.

Practical rule of thumb used by gold plant metallurgists:

Two practical notes. First, the cut matters as much as the nominal mesh: ask for the sieve distribution (percent retained on 6, 8, 12 mesh and the pan fraction) rather than accepting the words "6x12". Second, in column duty, platelet content - flat, flaky particles that pass screening but pack badly - should be controlled. Our production is water-polished and de-flaked, with platelet content held below 1.5%, because flakes cause channelling and break into fines faster.

2. Iodine number: what it measures, and what it does not

The iodine number is the standard proxy for micropore adsorption capacity, expressed as milligrams of iodine adsorbed per gram of carbon. It is run to ASTM D4607 internationally and GB/T 12496.8 in China. Coconut shell carbon is naturally microporous, which is why it dominates gold recovery: the gold cyanide complex Au(CN)2- is a small, linear ion that fits the micropore network of a coconut shell char almost perfectly.

Our gold-grade material is specified at iodine 1000-1300 mg/g depending on the ordered grade, with a tolerance of plus or minus 50 mg/g. A common gold-grade order sits around 1050 mg/g.

What the iodine number does not tell you:

• Kinetics. Iodine is an equilibrium lab test run over a fixed contact time. It says nothing about how fast your carbon loads gold in tank 5 of a CIL train. That is what the R-value (adsorption rate) measures.
• Gold capacity. The K-value is the gold-specific adsorption capacity (mg Au per g carbon), tested per GB/T 32992-2016. A coconut carbon with iodine 1050 can easily out-load a coal carbon with iodine 1100, because pore size distribution matters more than total number.
• Carbon loading. Iodine number is not a gold number. It does not tell you the carbon loading - the gold actually held on the carbon, reported as g Au per tonne of carbon - which depends on solution grade, contact time, carbon concentration and competing ions.
• Attrition and abrasion resistance. Iodine number is silent on what happens to the particle after 100 pump transfers; abrasion resistance is measured separately as ball-pan hardness.
• Ash and leachable matter. High ash means mineral content that contributes nothing to adsorption and can add to scale and fouling in the elution circuit.

If you want the two numbers closest to plant performance, ask your supplier for K-value and R-value on the actual batch (GB/T 32992-2016) alongside the COA, rather than for a higher iodine number.

3. Hardness: the number that protects your gold balance

Hardness - abrasion resistance measured by the ball-pan method, ASTM D3802 / GB/T 12496.6 - is the specification that decides how much carbon and how much gold leave your circuit as fines. Typical values:

Why this matters in commercial terms: fines below your screen aperture are not recoverable. They go to tails with the gold they carry, and the gold is only recovered if the plant has a fines recovery route. Carbon attrition in service is commonly of the order of 1-3% of the charge per elution and regeneration cycle - over a campaign that is a continuous carbon top-up cost, and a continuous small gold loss. A 98% hardness coconut carbon and a 92% hardness carbon can have identical COAs on iodine and still differ by tonnes of annual carbon consumption.

Three specification details that support hardness in practice:

• Apparent density 0.45-0.52 g/cm3 (GB/T 12496.1 / ASTM D2854). Density drives how many kilograms actually fit in your adsorption tank or column. A 10 m3 tank holds roughly 4,500 kg at 0.45 g/cm3 and 5,200 kg at 0.52 g/cm3 - a difference of about 700 kg, or 15%, that changes both your carbon inventory and your loading calculation. Always quote tank volume and ask the supplier to calculate the charge mass from measured density.
• Low ash (at most 5%). Ash does not adsorb gold and adds to elution circuit fouling.
• Moisture at most 5% as packed. Carbon is bought and shipped by weight; moisture is weight you pay for and do not adsorb with.

4. The 6x12 gold-grade specification table

Runtime-readable takeaway: anything you cannot see on a batch COA with a method column is not a specification - it is marketing.

5. Reading the COA that comes with the batch

Most sourcing failures in gold carbon are verification failures, not quality failures. A supplier brochure says "high hardness"; the delivered lot may not. When the container lands in Dakar, Abidjan or Tema, the COA is the only document that ties the numbers you ordered to the material you received.

A gold-grade COA should contain:

1. Lot or batch number, production date and test date - matched to the packing list and to the bags in the container.
2. Measured values, not ranges from a brochure, with a pass/fail column against the ordered specification.
3. Test methods next to each value (in our case GB/T 12496 series with ASTM equivalents).
4. Iodine, hardness, ash, moisture at minimum.
5. Sieve distribution - percent retained on 6, 8, 12 mesh and pan - so you can see how much fine material is in the lot, not just the nominal mesh.
6. Platelet content for column and CIC duty.
7. Optional but valuable: K-value and R-value per GB/T 32992-2016, and a third-party verification (SGS, Intertek, or a nominated lab) at the load port if your internal approval requires it.

Documents that normally travel with a coconut shell activated carbon shipment: COA, packing list, safety data sheet (activated carbon is not classified as a dangerous good for sea freight), certificate of origin, and - depending on the importing country - a phytosanitary or fumigation statement for the wooden pallets. A sample COA is published on our site so buyers can see the exact format before ordering: COA sample for gold carbon. For a deeper checklist, see how to verify a gold carbon COA.

6. How 6x12 carbon behaves in CIP, CIL and CIC

CIP (carbon-in-pulp). Leaching and adsorption are separate. Carbon moves counter-current through the adsorption tanks and is screened between stages. The duty is moderate abrasion with strict screen retention, so hardness and a clean 6x12 cut are the priority. Carbon concentration is typically in the 10-25 g/L range, adjusted by the metallurgist against the adsorption profile across the tank train.

CIL (carbon-in-leach). Leaching and adsorption happen in the same tank. Carbon residence time in pulp is longer, agitation is continuous, and the carbon sees more abrasion. For preg-robbing or slow-leaching ores CIL is often the right choice, but it is also the circuit where a low-hardness carbon costs the most.

CIC / column. The carbon never contacts pulp, only clarified pregnant solution. Abrasion is lower, but pressure drop, flow distribution and platelet content matter more, and 8x16 mesh is often preferred in smaller or short beds. The comparison of the three circuits in specification terms is covered in CIC vs CIP vs CIL.

Whatever the circuit, the two numbers to watch monthly are carbon concentration in the tanks and the fines content of the tailings screen undersize. Rising fines with stable iodine means an attrition problem, not a capacity problem - and attrition is a hardness and handling question, not a pore structure question.

7. Purchase order checklist

Put these on the PO rather than in an email footnote:

• Mesh cut with sieve distribution (6x12 mesh, or 8x16 for column duty).
• Iodine number with the tolerance (for example 1050 mg/g, plus or minus 50).
• Hardness minimum with the method (at least 98%, ASTM D3802 / GB/T 12496.6).
• Platelet content maximum (below 1.5%) for column duty.
• Apparent density range (0.45-0.52 g/cm3) with method.
• Moisture and ash maxima as packed.
• COA per batch, lot-specific, with methods - and the right to reject on the COA.
• Sample approval before shipment: 1-2 kg composite sample from the production lot for your own lab.
• Packaging: 25 kg bags on pallets, or 500/1000 kg jumbo bags with liner, plus moisture protection for humid or long-sea-freight destinations.
• Loading and delivery terms (FOB or CIF), port of discharge and any third-party inspection requirement.

A supplier who can answer all ten in writing, with a COA format you have seen in advance, is a supplier whose material you can defend in an internal approval. Our specification sheet is published here: gold carbon specification sheet.

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